1. Field of the Invention
The present invention relates to a positioning servomechanism. More particularly, the present invention relates to a positioning servomechanism operating between endstops, where a high speed collision with the endstops would damage the servomechanism itself or the equipment being positioned. Still more particularly, the present invention relates to the style of servomechanism which is used to position a radially mobile transducer over data storage tracks in a disc file. In greatest particularity, the present invention relates to an externally controlled servomechanism in a disc file where provision is made to avoid damage in the event of the external controller, or the servomechanism itself, developing a fault condition.
2. The Prior Art
The use of servomechanisms to position radially mobile read/write heads over data storage tracks, on discs in disc files is a well known art. The head is moved between endstops representing the least and greatest radii of its range. The read/write head is generally fragile, and is mounted on flimsy flexures. In the event of the servomechanism causing the head to collide violently with either of its endstop positions, the resultant short stopping distance, with attendantly high deceleration forces, may cause damage to the head, the head flexures, or the servomechanism actuator itself. In addition, there is usually provided a head unloading ramp, up which the head is parked when removed from the surface of the disc. Collision of the head with this ramp at high speed places the head in extreme risk of sustaining damage.
The trend towards higher performance disc files has resulted in the abandoning of relatively safe but slow stepping motor head positioning servomechanisms in favour of faster magnetic linear or rotary actuators, where a force proportional to current positions the head under the control of positional feedback information and externally applied demand and control signals. Should any fault develop in the servomechanism or the equipment providing demand and control signals, then there is a risk that the actuator will be accelerated uncontrolably towards one or other of its endstops. The relatively high cost of heads renders an original fault, most probably the result of a low cost failure, an unexpectedly costly fault to repair by necessitating the replacement of heads.
It is also the trend that microprocessors are used to command and control head positioning servomechanisms. Should malfunction or external influence disrupt the operation of the microprocessor, it is quite capable of entering a series of random, uncontrolled states in which commands may be sent to the servomechanism which, if obeyed, would result in damage and destruction.
It is therefore desirable that a method and apparatus be provided whereby the drive signals resulting from a fault condition in either the servomechanism itself or the external controller thereto may be overridden in the event of their being likely to cause damage, and the action of the servomechanism rendered harmless.